UV-curable resin, when exposed to UV, forms oligomers and monomers having UV-reactive groups. The resulting polymer is an insoluble material having a highly dense network as well as good heat resistance and chemical stability and superior optical and mechanical properties. Since the UV curing is accomplished in a relatively short time, it consumes less energy and provides good productivity thus being widely used in various fields. In addition, it has many other advantages that it is environment friendly, by not using any solvent, providing convenience by requiring only a small workplace, etc.
In general, the UV-reactive (meth)acrylate groups of monomers and oligomers may be crosslinked by a free radical chain mechanism by a photoinitiator. When exposed to UV or electron beam (EB), they turn to free radicals by the photoinitiator at a lower activation energy.
The photoinitiator has a disadvantage that it raises a safety issue. In fact, during or after the preparation of the photoinitiator, there is produced volatile, low-molecular-weight decomposition product, which can be easily absorbed through the respiratory organs or skin of workers, thereby affecting their health conditions.
Several solutions to this problem have been introduced. For example, there was an attempt to solve the problem by using a (meth)acrylate resin having a self-photoinitiating ability.
U.S. Pat. No. 5,565,525 (Nippon Paint, Japan, 1994) discloses a Michael addition product which is a UV-curable resin having four acryl functional groups prepared from a β-dicarbonyl Michael donor and a multifunctional acrylate Michael acceptor. It is prepared via a Michael addition reaction in the presence of an epoxide residue and a quaternary salt as catalyst. Although crosslinking is achieved without a photoinitiator, surface properties may not be good because of steric hindrance and improper acryl crosslinking density.
U.S. Pat. Appl. Pub. No. 2005-0261388 (Ashland, 2005) discloses a liquid, uncrosslinked, UV-curable Michael addition product having four acryl functional groups prepared from a multifunctional acrylate Michael acceptor and an optically active β-dicarbonyl Michael donor substituted with a photoinitiator. When exposed to UV, the resulting oligomer facilitates addition polymerization of the acryl groups through self-photoinitiation and improvement of optical activity by the introduced photoinitiator. Although viscosity is low and curing reactivity is superior, surface properties are not greatly improved because of improper acryl crosslinking density.
The self-photoinitiating (meth)acrylate oligomer prepared by Michael addition does not provide a desired effect when considering the additional preparation processes and cost. It is because the Michael acceptor has low acrylic functionality. Accordingly, it may be still required to add a volatile, low-molecular-weight multifunctional monomer to improve crosslinkability of the composition, which may negatively affect the reactivity and final property, as well as a photoinitiator.
Thus, various technical developments are underway to allow preparation of a photocurable (meth)acrylate resin in a safe and environment-friendly way without using an additional photoinitiator. Especially, techniques of preparing a (meth)acrylate oligomer having a built-in photoinitiator using no photoinitiator are presented.
In 1997, Ashland developed and patented a self-photoinitiating resin having 8 acryl functional groups from a multifunctional acrylate Michael acceptor and a β-dicarbonyl diacetoacetate Michael donor (U.S. Pat. Nos. 5,945,489 and 6,025,410). In 2009, Chokwang Paint developed a photocurable (meth)acrylate resin having four acryl functional groups through addition of a hydroxyacrylate Michael acceptor to a β-dicarbonyl donor followed by urethane bonding, which is capable of photocuring in the absence of a photoinitiator (Korean Patent No. 10-0903209).
U.S. Pat. Appl. Pub. No. 0004815 A1 (Ashland, 2007) discloses a self-photoinitiating multifunctional urethane acrylate having four acryl functional groups prepared from a Michael addition reaction with a β-dicarbonyl Michael donor, wherein an amine-based synergist is used to provide improved photocuring reactivity.
However, in order to improve effective performance, a Michael addition product having more acryl functional groups is required.